Most liquid crystal displays (LCDs) are made from a sandwich of liquid crystal (LC) between two glass substrates coated with a transparent conductor. The glass substrates are generally held together to a predetermined gap using an epoxy-based gasket at the edge. For glass LCDs, two big challenges remain; namely, flexibility and durability. These problems can be addressed by using plastic substrates. However, the need remains for an effective edge seal to prevent the two plastic substrates from delaminating.
One advantage in using plastic to make LCDs is to make the display in a roll-to-roll process. For roll-to-roll processing to be effective, the liquid crystal material should be filled between two spaced substrates and prevented from flowing by an encapsulation process. Liquid crystal can be encapsulated using microcups (or well-defined polymer walls), emulsification, Thermally Induced Phase Separation (TIPS), Solvent Induced Phase Separation (SIPS), or Polymerization Induced Phase Separation (PIPS). With PIPS, for example, the droplets of liquid crystal are excluded from the bulk via phase separation. As polymeric chains grow in molecular weight the liquid crystal becomes encapsulated into micron-sized droplets by solid polymer walls. Once encapsulated, the liquid crystal cannot flow between the substrates or leak out if the substrates are cut. Oftentimes the polymer walls created during the encapsulation process are not very strong since they can have a width on the scale of micron to sub-micron. Due to the thin polymer walls and the fact that there is very little solid material to hold the substrates together, delamination of the two bounding substrates is very easy to nucleate at the edges of the LCD. Therefore, an edge seal mechanism is required to prevent delamination.
An LCD can be edge sealed in a number of ways. As mentioned above, one may use an internal gasket (e.g., composed of epoxy or photo-resin), an adhesive tape, or an external gasket such as silicone. The internal gasket is very effective for edge sealing and is the dominant technology used in the LCD industry because of a strong bond between the substrates and low gas (moisture) permeability. However, it is best suited to a batch process, as roll-to-roll manufacturing a gasket seal is technologically challenging in that both the LC and the resin seal must be applied (as liquids) simultaneously. External adhesive tape forms a weaker bond, is less mechanically robust and is generally quite permeable and susceptible to water-damage. An external gasket such as silicone is an effective moisture barrier as it has low gas permeability. However, it is not very mechanically robust, as it tears easily. In the case of external edge seals such as adhesive tape and external gaskets, one must first make the display on a roll, singulate (or cut) the display, and then apply the edge seal.
It is well known that infrared lasers can cut and weld plastic materials (U.S. Pat. Nos. 4,069,080 and 4,224,096). Laser welding two thermoplastic substrates together requires that the two plastic pieces absorb infra-red light, heat-up, and flow together to create a strong bond. A limitation of conventional laser welding is the understanding that only pure thermoplastic materials in contact with one another can be laser welded because impurities in the area to be bonded can contaminate the bond and cause its failure.
This disclosure enables using lasers to cut and weld substrates in the formation of displays even in the presence of liquid crystal or other materials as impurities.